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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var money = 100;
        while (true) {
            var num = prompt("请输入你要进行的操作:" + "\n" + "1.存钱" + "\n" + "2.取钱" + "\n" + "3.显示余额" + "\n" + "4.退出")
            switch (parseInt(num)) {
                case 1:
                    var number = prompt("请输入你存钱的数目:");
                    money += parseInt(number);
                    alert("你现在的余额为:" + money);
                    continue;
                case 2:
                    var save = prompt("请输入你取钱的数目:");
                    money -= parseInt(save);
                    alert("你现在的余额为:" + money);
                    continue;
                case 3:
                    alert("你现在的余额为:" + money);
                    continue;
                case 4:
                    alert("您正在退出");
                    break;
            }
            if (parseInt(num) == 4)
                break;
        }
    </script>
</head>

<body>

</body>

</html>